Marymoor's off-leash dog park is my favorite on the weekends. Just don't forget to bring a dollar to pay for parking.
I love this dog park because it's huge and because it's designed so that humans can get their exercise too. The gravel and bark paths rim and cut through the off-leash area in such a way that you can amble for a whole hour without hitting the same path twice. Meanwhile, your canine companions can race ahead, explore underbrush, swim in the creek, or romp on the grassy areas.
I enjoy walking alongside the creek, then cutting through the marshy area before descending on the lawn which comprises the bulk of the park. You see a lot of games of fetch here, but a lot of dogs just use this opportunity to race around at top speed. My dogs only ever run this fast and hard when they're at my in-laws' ranch.
You almost never see dog-on-dog violence here and I think it's because there's so much open space. It never feels crowded even though there are always tons of people and dogs. Lots of open space means there aren't a ton of trees or benches. Despite the fact that there are hardly any trees for cover, people come here in all kinds of weather, even pouring-down rain. On those occasions, the coffee/snack truck might not be in the parking lot, but if it is, you'd better get coffee to warm your hands.
I occasionally see people keep their dogs on leashes here because the park is conducive to dog-walking, but I think that's just plain cruel. Some people have their itty-bitty dogs on leashes so they can swoop them up in case there's drama, but I think that's kind of silly too. Most dogs here are super-friendly. The ones that aren't as friendly can easily explore independently if they don't feel like playing with others. The variety of dogs here is better than any other dog park. (I think a lot of looky-loos come here to scope out potential breeds.) Also, there is never any shortage of poop-scooping bags thanks to the local grocery stores.